1. Field of the Invention
The present invention is directed to a MR transmission or reception system (MR resonator system) with a transmitter or with a receiver provided with a pre-amplifier that is connected to a transmission or reception coil, possibly via mixers and matching elements.
2. Description of the Prior Art
Known transmitter assemblies (the same is true for known reception assemblies) for MR systems are composed of a transmitter, transmission and matching elements and the transmission coil. Two matching elements are required that transform the transmission energy from the transmitter output impedance on to the characteristic impedance of the transmission channel and, from the latter, to the impedance of the (resonant) transmission coil. A high radio-frequency energy density occurs in the transmission elements. Moreover, an individual drive of the individual coil elements is possible only with high component outlay.
An object of the present invention is to design an MR transmission or reception (resonator) system of the type initially described wherein the difficulties with the matchings of the transmission or reception coil are avoided with a simple structure and such that a simple, individual drive of the individual coil elements is possible.
This object is inventively achieved in an MR transmission or reception system wherein at least parts of the transmitter or receiver and of the appertaining drive elements are integrated into the transmission or reception coil. As a result of this integration of the transmitter or the receiver into the coil, the matching and transmission elements, including the transmission cable and the transformation elements for impedance matching to the coils, are largely or entirely eliminated. Moreover, difficulties associated with an excessively high radio-frequency energy density in the transmission elements no longer occur due to their elimination.
In an embodiment of the invention, together with the integration of the transmitter or of the receiver into the coil, the transmitter is split into a number of modules integrated into the coil branches. This not only makes the system symmetrical but also enables an individual drive of the individual coil elements.
The individual coil branches can be digitally drivable, and the switch elements driving the individual coil branches should be capable of being driven time-delayed, so that, for example, a magnetic rotating field can be achieved in the target volume of the coil (circular polarization).
In order to achieve the delay, in an embodiment of the invention the analog delay elements are inserted either into the drive lines of the individual coil branches or digital delay devices are inserted into the drive lines of the individual coil branches, these being fashioned, for example, as shift registers.
For reducing the switching losses, in another embodiment of the invention the individual coil branches can be operated half-resonant (i.e. resonant with low quality, whereby each half-wave or full-wave is individually influenced). In this context it is expedient for the switch elements driving the individual coil branches to switch at the zero-axis crossing of the voltage or of the current (zero voltage switches ZVS or zero current switches ZCS).
For modulating the transmission power, the supply voltage can be modulated, such s with pulse-width modulation or a resonant packet control, i.e. bursts of half-waves.
The individual coil branches as noted above can be driven separately or differently in order, for example, to improve the B1 homogeneity or in order to spatially focus the target field (for example, SAR reduction, i.e. reduction of the specific absorption rate of the patient in order to avoid overheating due to the RF energy).
Although the application of the invention to transmission coils is specifically discussed above, analogous integration and design apply to reception coils with inventively integrated receivers.